System and method for optimizing flux regulation in electric motors

ABSTRACT

A system and method for dynamically optimizing flux levels in electric motors based on estimated torque. Motor parameters and motor equations are used to estimate operating characteristics and to set current and voltage limits which define an optimal flux operating range for a given speed and torque of the motor. A slope of a linear flux gain is determined within the defined operating range at different speeds of the motor. The determined slopes for the different speeds are saved in a memory element. A control element determines and achieves an optimal flux level for the motor by accessing the table to identify a specific slope which corresponds to an actual speed of the motor, multiplying the slope by the estimated torque and adding an offset value to determine a phase current component value associated with the optimal flux level, and applying the determined phase current component value to the motor.

RELATED APPLICATION

The present U.S. non-provisional patent application is a continuationand claims priority benefit of a prior-filed U.S. non-provisional patentapplication having the same title, Ser. No. 15/337,672, filed Oct. 28,2016, and of a prior-filed U.S. non-provisional patent applicationhaving the same title, Ser. No. 14/830,583, filed Aug. 19, 2015. Theentire contents of the identified prior-filed applications are herebyincorporated by reference into the present application as if fully setforth herein.

FIELD

The present invention relates to systems and methods for controlling theoperation of electric motors, and, more particularly, to a system andmethod for optimizing flux regulation in electric motors.

BACKGROUND

Electric induction motors are normally designed to provide rated load atrated speed. Simple control systems use voltage/frequency (V/F) curvesthat generate constant torque up to the knee of the curve. However, mostsuch V/F curves are each tuned to a specific load and constant flux, yetmany electric motors experience varying loads. If the load is less thanthe rated torque, excess flux is delivered resulting in less efficiency.Furthermore, electric motor systems designed for maximum load draw morepower and generate more heat which reduces the reliability of thosesystems.

Optimal flux regulation provides a method of controlling efficiency.Normally the best efficiency is achieved by balancing copper and corepower losses. When both reach the same power levels, the electric motoris generally running at the most efficient operating point. However,this is not the case at low torque loads. One method of controllingefficiency monitors estimated real power and adjusts the flux until thereal power is at the lowest point. A problem occurs if the operatingrange is small due to current and voltage limits. Also, the startingflux point must be within the operating range or the electric motor willnot generate the necessary torque. The current and voltage limit is setby the control hardware. The current limit is based on the control'sinverter capability, and exceeding the current limit may damage theinverter. The voltage limit is based on the direct current (DC) supply.Controls with a power factor control (PFC) will maintain a relativelyconsistent DC voltage, while controls without a PFC will rely on thealternating current (AC) voltage supplying the control and must cut offat a predetermined low DC voltage level. Many controls now use vectorcontrol that separates the phase current (I) into two vector currentcomponents: The I_(d) current vector regulates the flux, while the I_(q)current regulates the torque.

This background discussion is intended to provide information related tothe present invention which is not necessarily prior art.

SUMMARY

Embodiments of the present invention solve the above-described and otherproblems and limitations by providing a system and method fordynamically optimizing flux levels in electric motors based on estimatedtorque, and thereby improving efficiency, decreasing operatingtemperature, and increasing reliability.

In a first embodiment, a method of the present invention may proceedsubstantially as follows. A set of motor parameters and a set of motorequations may be used to estimate a slip, a stator frequency, a torque,and a power loss and to set a current limit and a voltage limit whichdefine an optimal flux operating range for a given speed and torque ofthe motor. A slope of a linear flux gain may be determined within theoptimal flux operating range at a plurality of different speeds of themotor. The slope of the linear flux gain for each different speed may besaved in an electronic memory element. An electronic control element maydetermine an optimal flux level for the motor by accessing the tablestored in the memory element to identify a specific slope of the linearflux gain which corresponds to an actual speed of the motor, multiplyingthe slope of the linear flux gain by the estimated torque and adding anoffset value to determine a phase current component value associatedwith the optimal flux level, and applying the determined phase currentcomponent value to the motor.

In a second embodiment, a system of the present invention may broadlycomprise an electric motor and a motor control subsystem. The motor mayhave a shaft and may be configured to create a torque on the shaft todrive a load. The motor control subsystem may be configured to controloperation of the motor, and may include an electronic memory element andan electronic control element. The memory element may contain a table ofslopes of a linear flux gain for a plurality of different speeds. Thetable may be created by using a set of motor parameters and a set ofmotor equations to estimate a slip, a stator frequency, a torque, and apower loss and to set a current limit and a voltage limit which definean optimal flux operating range for a given speed and torque of theelectric motor, determining a slope of a linear flux gain within theoptimal flux operating range at a plurality of different speeds of theelectric motor, and saving in the memory element the slope of the linearflux gain for each different speed. The control element may be incommunication with the memory element and configured to determine andachieve an optimal flux level for the motor by accessing the memoryelement to identify a specific slope of the linear flux gain whichcorresponds to an actual speed of the motor, multiplying the slope ofthe linear flux gain by the estimated torque and adding an offset valueto determine a phase current component value associated with the optimalflux level, and applying the determined phase current component value tothe motor.

Various implementations of each of the foregoing embodiments may includeany one or more of the following additional features. The motor may be avariable speed, alternating current induction motor. The load may beselected from among fans, pumps, blowers, rotating drums, components ofclothes washers or clothes dryers, components of ovens, components ofheating and air-conditioning units, and components of residential orcommercial machines. The current limit may correspond to a lower fluxlimit based on a torque load level for a given speed of the motor, andthe voltage limit corresponds to an upper flux limit based on the torqueload level which results in a lower phase current torque component. Themotor equations may include a slip equation, a voltage equation, atorque equation, and a power equation. The offset value may be a commonoffset value for the plurality of different speeds and is based on thephase current at a lowest torque point. The optimal flux may bedetermined by the torque resulting in a lowest power level. The methodand/or system may further include adjusting the set of motor parametersbased on a saturation of the motor and/or on a temperature of the motor.

This summary is not intended to identify essential features of thepresent invention, and is not intended to be used to limit the scope ofthe claims. These and other aspects of the present invention aredescribed below in greater detail.

DRAWINGS

Embodiments of the present invention are described in detail below withreference to the attached drawing figures, wherein:

FIG. 1 is an exploded depiction of an embodiment of an electric motorsystem of the present invention, wherein the electric motor system isshown configured to drive a load;

FIG. 2 is a plot if I versus I_(d), and showing an operating range ofI_(d) based on voltage and current limits;

FIG. 3 is a plot of Id versus torque at 4000 revolutions per minute(rpm);

FIG. 4 is a plot of Id versus torque at 3000 rpm;

FIG. 5 is a plot of Id versus torque at 2000 rpm;

FIG. 6 is a plot of Id versus torque at 1000 rpm;

FIG. 7 is a plot of Id versus torque at 600 rpm; and

FIG. 8 is a flowchart of an embodiment of a method of the presentinvention.

The figures are not intended to limit the present invention to thespecific embodiments they depict. The drawings are not necessarily toscale.

DETAILED DESCRIPTION

The following detailed description of embodiments of the inventionreferences the accompanying figures. The embodiments are intended todescribe aspects of the invention in sufficient detail to enable thosewith ordinary skill in the art to practice the invention. Otherembodiments may be utilized and changes may be made without departingfrom the scope of the claims. The following description is, therefore,not limiting. The scope of the present invention is defined only by theappended claims, along with the full scope of equivalents to which suchclaims are entitled.

In this description, references to “one embodiment”, “an embodiment”, or“embodiments” mean that the feature or features referred to are includedin at least one embodiment of the invention. Separate references to “oneembodiment”, “an embodiment”, or “embodiments” in this description donot necessarily refer to the same embodiment and are not mutuallyexclusive unless so stated. Specifically, a feature, structure, act,etc. described in one embodiment may also be included in otherembodiments, but is not necessarily included. Thus, particularimplementations of the present invention can include a variety ofcombinations and/or integrations of the embodiments described herein.

Broadly characterized, the present invention provides a system andmethod for dynamically optimizing flux levels in electric motors basedon estimated torque, and thereby improving efficiency, decreasingoperating temperature, and increasing reliability. Referring to the FIG.1, an embodiment of an electric motor system 20 is shown broadlyincluding an electric motor 22 having a stator 24, a stator winding 26,a rotor 28, and a shaft 30 configured to drive a load 32, and a motorcontrol subsystem 34 including an electronic control element 36 and anelectronic memory element 38. The electric motor 22 may be a variablespeed electric motor. For example, the electric motor 22 may be amulti-phase, multi-pole AC induction motor. The stator 24, statorwinding 26, and rotor 28 may cooperate in an otherwise substantiallyconventional manner to turn the shaft 30 and thereby drive the load 32.The load 32 may be, e.g., a fan, a pump, a blower, a rotating drum, acomponent of a clothes washer or clothes dryer, a component of an oven,a component of a heating and air-conditioning unit, or a component of aresidential or commercial machine.

The motor control subsystem 34 may be broadly configured to controloperation of the electric motor 22. The various components of the motorcontrol subsystem 34 may be implemented in hardware and/or software, andmay be configured to receive input signals from a user interface and/orone or more sensors and to generate control signals based on such inputto control operation of the electric motor 22. In one implementation,the motor control subsystem 34 may receive AC power from an AC powersource, and may condition the AC power to drive the electric motor 22 inaccordance with a speed command specifying a speed at which the electricmotor 22 is to be run.

In particular, the control element 36 may be any suitable controltechnology configured to receive a power input, user commands, and/orsensor data, and based thereon control operation of the electric motor22. The control element 36 may be in communication with the memoryelement 38. The memory element 38 may be any suitable non-transitoryelectronic or other memory technology configured to store data forsubsequent access by the control element 36. In particular, the memoryelement 36 may store data and/or one or more computer programs used bythe control element 36 in controlling operation of the electric motor22.

A torque available at the shaft 30 is determined by the flux acting onthe stator winding 26 and the distance of that force from the center ofrotation. The flux is determined by I_(d) flowing through the statorwindings 26 and a strength of a plurality of field magnet components ofthe electric motor 22. More specifically,

$T = {\frac{3}{2} \times {pp} \times \frac{L_{m}^{2}}{L_{r}} \times I_{q} \times I_{d}}$where T=torque, pp=pole pairs, L_(m)=phase magnetizing inductance,L_(r)=rotor phase inductance, I_(d)=magnetizing current, andI_(q)=torque producing current. When excited by a given voltage andcurrent, the electric motor 22 may exhibit a speed/torque curve. Theload 32 on the shaft 30 causes the rotor 28 to slow, which creates slip.Thus, slip is the difference between the stator magnetic field speed andthe rotor speed. A slip level associated with the knee of the V/F curveresults in maximum torque and power transfer from the electric motor 22.Thus, this is normally the preferred place on the V/F curve to operatethe electric motor 22. Vector control (or slip control) may be used tokeep the electric motor 22 operating at this optimum point on the V/Fcurve. Vector control may be implemented by the motor control subsystem34 using a mathematical model of the electric motor 22 stored in thememory element 38 and a position transducer (not shown) on the electricmotor 22 to indicate a position of the rotor 28. The mathematical modelallows the control element 36 to determine a speed/torque curve for theelectric motor 22 given any applied voltage and frequency, which allowsthe motor control subsystem 34 to control the slip in the electric motor22 to keep it operating at the knee of the speed/torque curve.

The present invention provides a software-based control solution whichmay be stored on the memory element 38 and executed by the controlelement 36 for dynamically optimizing flux levels based on estimatedtorque. Broadly, an upper flux limit and a lower flux limit for a givenspeed determine the flux range at that speed. The upper flux limit maybe based on a rated torque at a given speed. The motor controlsubsystem's 34 hardware limits may reduce flux more when reaching a busvoltage limit and/or a bus current limit of the hardware. The lower fluxlimit may be based on a free shaft or lowest torque load level. Optimalflux may be determined by the torque resulting in the lowest phasecurrent. The phase current may be sensed using shunts on each phase.Because estimated torque is a function of the phase current components,I_(d) and I_(q), optimal flux can be based on the electric motor'soperating torque level. This update may occur slower than the severalrotor time constants. This is an improvement over other methods thatrequire the system to converge on the steady state value.

In one implementation, the optimal flux may be determined by simulationby establishing the electric motor parameters in the slip, voltage,torque and power equations. More specifically, given a set of electricmotor parameters associated with the electric motor 22, a set ofelectric motor equations may be used to estimate slip, stator frequency,torque, and power losses. These equations may be used to set the controlcurrent and voltage limit that shows the flux operating range (the I_(d)range) at a given speed and torque. FIG. 2 shows an exemplary plot 50 ofI versus I_(d), and showing an operating range of I_(d) based on givenvoltage and current limits for one speed/torque point. In this range,the lowest power level may be determined. Several speed points may beused to generate the optimal flux table. FIGS. 3-7 are exemplary plots52,54,56,58,60 of optimal Id at each speed/torque point for 4000, 3000,2000, 1000, and 600 rpm, respectively. The end result is a close tolinear flux gain that is multiplied by the estimated torque to determinethe target I_(d) current representing the optimal flux. An exemplarytable based on FIGS. 2-7, which may be stored in the memory element 38and referred to by the control element 36 in controlling operation ofthe electric motor 22, and which may be set to the lowest power pointfor each target torque is shown below, in which I_(d)=(estimatedtorque×kslope)+koffset, wherein koffset=>I_(d) at table's lowest torquepoint, and kslope=>I_(d) slope over table torque range. In this example,koffset for all speed ranges set to 1.4.

Target Flux Table Speed (rpm) kslope 4000 0.2533 3450 0.3100 3000 0.32932500 0.3293 2000 0.3874 1500 0.4262 1000 0.5424 600 0.6393

The control element 36 may use such a table of interpolated kslopevalues to determine optimal flux by, for a given speed, reading kslopefrom the table, multiplying the estimated torque by the interpolatedkslope, and then adding the common koffset for the final I_(d) currentused to create the flux.

The set of electric motor parameters may be adjusted due to saturationand temperature. High currents will generally cause the inductance todecrease, so the inductance parameter may be set as a function ofcurrent. Resistance changes with temperature and becomes more of aproblem at low speeds, so the resistance parameter may be set as afunction of temperature.

In another implementation, the optimal flux may be determinediteratively, i.e., by converging the algorithm. More specifically, thetorque equation may be used to determine the lowest I_(d) and I_(q)currents iteratively. Because speed regulators normally generate atorque command output, this torque command or estimated torque load canbe used to determine the optimal flux. Many motor control drives now usevector control algorithms that independently control flux- andtorque-producing currents. Flux is controlled based on Id current andtorque is controlled based on Iq current. The present invention involvessetting the optimal Id current that results in optimal flux.

For stability reasons, the target flux may be regulated slower than thespeed or torque regulators. It is assumed that the target flux isupdated slower than the rotor time constant.

Referring also to FIG. 8 an embodiment of the method 100 of the presentinvention may proceed substantially as follows. A set of electric motorparameters may be adjusted based on the saturation of the electric motor22 and/or the temperature of the electric motor 22, as shown in step102. The set of electric motor parameters and the set of electric motorequations may be used to estimate such operating characteristics as aslip, a stator frequency, a torque, and a power loss and to set acurrent limit and a voltage limit which define an optimal flux operatingrange for a given speed and torque of the electric motor 22, as shown instep 104. The current limit may correspond to a lower flux limit basedon a torque load level for a given speed of the electric motor 22, andthe voltage limit may correspond to an upper flux limit based on thetorque load level which results in a lower phase current torquecomponent. A slope of a linear flux gain may be determined within theoptimal flux operating range at a plurality of different speeds of theelectric motor 22, as shown in step 106. The determined slope of thelinear flux gain for each different speed may be saved in the memoryelement 38, as shown in step 108. The control element 36 may determinethe optimal flux level for the electric motor 22 by accessing the memoryelement 38 to identify a specific slope of the linear flux gain whichcorresponds to an actual speed of the electric motor, 22, as shown instep 110, multiplying the slope of the linear flux gain by the estimatedtorque and adding an offset value to determine a phase current componentvalue associated with the optimal flux level, as shown in step 112, andapplying the determined phase current component value to the electricmotor 22, as shown in step 114. The offset value may be a common offsetvalue for the plurality of different speeds and is based on the phasecurrent at a lowest torque point.

Thus, the present invention provides substantial advantages over theprior art, including that it dynamically optimizes flux levels inelectric motors based on estimated torque, and thereby improvingefficiency, decreasing operating temperature, and increasingreliability.

Although the invention has been described with reference to the one ormore embodiments illustrated in the figures, it is understood thatequivalents may be employed and substitutions made herein withoutdeparting from the scope of the invention as recited in the claims.

Having thus described one or more embodiments of the invention, what isclaimed as new and desired to be protected by Letters Patent includesthe following:
 1. An electric motor system for optimizing a flux levelin an electric motor configured to receive an input power and to drive aload, the electric motor system comprising: an electric motor having ashaft and configured to create a torque on the shaft to drive the load;and a motor control subsystem configured to control operation of theelectric motor, the motor control subsystem including— an electronicmemory element containing a table of linear flux gains for a pluralityof different speeds, wherein the table is created by— using a set ofmotor parameters and a set of motor equations to estimate a slip, astator frequency, and a torque, and defining an optimal flux operatingrange for a given speed and torque of the electric motor, determining alinear flux gain within the optimal flux operating range at a pluralityof different speeds of the electric motor, and saving in the electronicmemory element the linear flux gain for each different speed, and anelectronic control element in communication with the memory element andconfigured to determine and achieve an optimal flux level for theelectric motor at which the input power to the electric motor isminimized by— accessing the memory element to identify a specific linearflux gain which corresponds to an actual speed of the electric motor,multiplying the specific linear flux gain by the estimated torque andadding an offset value to determine a phase current component valueassociated with the optimal flux level, and applying the determinedphase current component value to the electric motor.
 2. The system asset forth in claim 1, wherein the electric motor is a variable speed,alternating current induction motor.
 3. The system as set forth in claim1, wherein the load is selected from the group consisting of: fans,pumps, blowers, rotating drums, components of clothes washers or clothesdryers, components of ovens, components of heating and air-conditioningunits, and components of residential or commercial machines.
 4. Thesystem as set forth in claim 1, wherein— the current limit correspondsto a lower flux limit based on a torque load level for a given speed ofthe electric motor; and the voltage limit corresponds to an upper fluxlimit based on the torque load level which results in a lower phasecurrent torque component.
 5. The system as set forth in claim 1, whereinthe offset value is a common offset value for the plurality of differentspeeds and is based on the phase current at a lowest torque point. 6.The system as set forth in claim 1, wherein the electronic controlelement is further configured to— adjust the set of motor parametersbased on a saturation of the electric motor; and adjust the set of motorparameters based on a temperature of the electric motor.
 7. Acomputer-implemented method for improving the functioning of a computerfor optimizing a flux level in an electric motor configured to drive aload, the computer-implemented method comprising: using a set of motorparameters and a set of motor equations to estimate a slip, a statorfrequency, and a torque, and defining an optimal flux operating rangefor a given speed and torque of the electric motor; determining a linearflux gain within the optimal flux operating range at a plurality ofdifferent speeds of the electric motor; saving in an electronic memoryelement of the computer the linear flux gain for each different speed;and determining in an electronic control element of the computer anoptimal flux level for the electric motor by— accessing the memoryelement to identify a specific linear flux gain which corresponds to anactual speed of the electric motor, multiplying the specific linear fluxgain by the estimated torque and adding an offset value to determine aphase current component value associated with the optimal flux level,and applying the determined phase current component value to theelectric motor.
 8. The computer-implemented method as set forth in claim7, wherein the electric motor is a variable speed, alternating currentinduction motor.
 9. The computer-implemented method as set forth inclaim 7, wherein the load is selected from the group consisting of:fans, pumps, blowers, rotating drums, components of clothes washers orclothes dryers, components of ovens, components of heating andair-conditioning units, and components of residential or commercialmachines.
 10. The computer-implemented method as set forth in claim 7,wherein— the current limit corresponds to a lower flux limit based on atorque load level for a given speed of the electric motor; and thevoltage limit corresponds to an upper flux limit based on the torqueload level which results in a lower phase current torque component. 11.The computer-implemented method as set forth in claim 7, wherein themotor equations include a slip equation, a voltage equation, and atorque equation.
 12. The computer-implemented method as set forth inclaim 7, wherein the offset value is a common offset value for theplurality of different speeds and is based on the phase current at alowest torque point.
 13. The computer-implemented method as set forth inclaim 7, wherein the optimal flux is determined by the torque resultingin a lowest power level.
 14. The computer-implemented method as setforth in claim 7, further including adjusting the set of motorparameters based on a saturation of the electric motor.
 15. Thecomputer-implemented method as set forth in claim 7, further includingadjusting the set of motor parameters based on a temperature of theelectric motor.
 16. A computer-implemented method improving thefunctioning of a computer for optimizing a flux level in an electricmotor, wherein the electric motor is a variable speed, alternatingcurrent induction motor configured to drive a load, thecomputer-implemented method comprising: using a set of motor parametersand a set of motor equations to estimate a slip, a stator frequency, anda torque, and defining an optimal flux operating range for a given speedand torque of the electric motor; determining a linear flux gain withinthe optimal flux operating range at a plurality of different speeds ofthe electric motor; saving in an electronic memory element of thecomputer the linear flux gain for each different speed; and determiningin an electronic control element of the computer an optimal flux levelfor the electric motor by— accessing the memory element to identify aspecific linear flux gain which corresponds to an actual speed of theelectric motor, multiplying the specific linear flux gain by theestimated torque and adding an offset value to determine a phase currentcomponent value associated with the optimal flux level, wherein theoffset value is a common offset value for the plurality of differentspeeds and is based on the phase current at a lowest torque point, andapplying the determined phase current component value to the electricmotor.
 17. The computer-implemented method as set forth in claim 16,wherein the load is selected from the group consisting of: fans, pumps,blowers, rotating drums, components of clothes washers or clothesdryers, components of ovens, components of heating and air-conditioningunits, and components of residential or commercial machines.
 18. Thecomputer-implemented method as set forth in claim 16, wherein— thecurrent limit corresponds to a lower flux limit based on a torque loadlevel for a given speed of the electric motor; and the voltage limitcorresponds to an upper flux limit based on the torque load level whichresults in a lower phase current torque component.
 19. Thecomputer-implemented method as set forth in claim 16, further includingadjusting the set of motor parameters based on a saturation of theelectric motor.
 20. The computer-implemented method as set forth inclaim 16, further including adjusting the set of motor parameters basedon a temperature of the electric motor.